The invention relates to the field of covering, and preventing the evaporation of an aqueous liquid, more particularly to a composition and a method for covering the sump in cavities leached, for example, out of salt-domes and serving to store gaseous or liquid, non-aqueous substances.
U.S. Pat. No. 4,403,887, which is hereby expressly made a part of the disclosure of the present invention, describes a method for covering cavities with a crude oil containing not more than 7% by weight of hydrocarbons having at the most 5 C-atoms in the molecule, the specific weight thereof amounting, at a temperature of 20.degree. C., to at the most 1.2 g/cm.sup.3, the said crude oil by applied to the sump in the form of a layer at least 1 cm in thickness. This method is used in cavities to store gases of all kinds, especially combustible gases such as natural gas, methane, propane, butane and mixtures thereof, mineral products, but also air. The storage media may be gaseous or liquid; they may also be partly gaseous and partly liquid.
The purpose of a crude-oil sump covering of this kind is largely to prevent the passage of water or water-vapour from the sump to the storage medium. The water-absorption of the storage medium may thereby be reduced to about one thousandth of the value obtained with an uncovered sump.
The said publication contains a calculation of the change in the dew-point of natural gas, based upon laboratory tests. The cavity was of normal dimensions, i.e. having a capacity of about 250 000 to 350 000 m.sup.3 and a surface area of about 900 to 1 100 m.sup.2. It is assumed that the gas remains in storage for about one year at a pressure of about 70 bars. The resulting dew-point changes are as follows:
______________________________________ with no sump-cover from -8.degree. C. to +38.degree. C. with a 10 mm sump-cover from -8.degree. C. to -6.degree. C. with a 250 mm sump-cover from -8.degree. C. to -7.degree. C. ______________________________________
A sump-cover of this kind eliminates the need for drying units which are otherwise required to lower the dew-point of the gas removed from storage, and possibly saturated with water-vapour, to such an extent that it can be transported by pipeline or used for some other purpose.
However, since the period of storage in cavities may also be substantially longer, there has hitherto been a reluctance to dispense completely with the costly drying units.